Sea ice displacement measured by ERS-1 SAR interferometry

SAR interferometry (InSAR) offers new possibilities to sea ice mechanics research where it can provide information about small relative movements and surface topography. The ambiguity between these features is a problem, but can in some cases be separated from each other using three or more images. Vertical topography maps are useful for examining ice ridges but the ice then has to be stationary which requires a very good control of the ice field. A potential application is to use the system to determine the topography of grounded ridges. Nearly stationary ice can be examined from the small relative movements and bounds can be set to horizontal shifts in the ice pack. A case study is presented from the Baltic Sea in late March 1992. The temperatures were below zero resulting in relatively high coherence and good fringe visibility in the SAR data, The ambiguity between ridge heights and shifts means that sub-mm movements may be interpreted as very high ridges (2-4 m for a measured ridge). The ice was nearly stationary as the stresses were below the yield limit under the low forcing conditions, and the forcing directions also indicated significant shear strength, With larger distance between sensor positions (i.e. baseline) or with no time lag between image acquisitions (for example from an airborne system), topographic maps over ice are possible and hence ice ridges should be possible to detect. On the other hand, repeat-pass InSAR with small baselines will result in high-quality displacement maps, Such observations will help assessing the ice rheology for sea ice.